Coal combustion remains a major source of energy in many parts of the world. The solid waste remaining after the combustion is called coal ash, which comprises fly ash--the fine particles collected from the combustion-off gas by electrostatic precipitators--and the ash remaining at the bottom of the combustion vessels. The volume of coal ash resulting from coal combustion world-wide is constantly on the rise, resulting in an ever increasing problem of disposing the waste without damaging the environment. In addition, the availability of the waste products at no cost and their being adjacent to sources of power and transport make them attractive as raw material for the extraction of various chemical products.
The chemical composition of coal ash varies as a function of the source and type of the coal. However, Al.sub.2 O.sub.3 and SiO.sub.2 are almost always major components of the ash, usually in the form of aluminosilicates and quartz. It would therefore be advantageous to use coal ash as a source for alumina and silica.
The major source of pure alumina today is bauxite, with the alumina generally being extracted by the Bayer process. As bauxite is in ample supply and the alumina can be extracted relatively easily and inexpensively, any alternate method for extracting alumina must be economically competitive with the Bayer process. In calculating the process cost, the savings in disposal and disposal site maintenance costs should be taken into account, as well as environmental considerations.
A number of processes for recovering aluminum from coal ash have been reported (Seeley, F. G., Canon, R. M. and McDowell, W. J., "Chemical Development of New processes for the Recovery of Resource Materials from Coal Ash", Oak Ridge National Laboratory, Contract W-7405-eng-26; Felker, K., et al, "Aluminum from Fly Ash", Chemtech 12(2):123-8 (1982)). These include direct acid leach methods, lime-sinter and lime-soda-sinter methods, a salt-soda sinter process and the Calsinter process.
Direct acid leaching (either single-stage or multi-stage) with HCl, HNO.sub.3, or H.sub.2 SO.sub.4 usually results in quite low recovery rates of Al (under 50%).
Lime and lime/soda sinter processes involve sintering coal wastes at 1200-1300.degree. C. with powdered limestone (CaCO.sub.3) or limestone and soda ash (Na.sub.2 CO.sub.3) to form calcium or sodium aluminates. The aluminates are then dissolved by leaching with Na.sub.2 CO.sub.3.
In the salt-soda sinter process, a NaCl--Na.sub.2 CO.sub.3 mixture is sintered with fly ash, quenched in a water leach, and then leached in a dilute HNO.sub.3 or H.sub.2 SO.sub.4 solution.
The Calsinter process (developed at Oak Ridge National Laboratory, Tennessee, U.S.A.) involves the combination of a CaSO.sub.4 --CaCO.sub.3 -fly ash sintering system and an acid leach with H.sub.2 SO.sub.4.
Recently, another method for recovery of alumina from alumino-silicates has been described in GB 2,205,558. In this method, the alumino-silicate is reacted with hydrated calcium and/or magnesium chloride, with or without a minor proportion of sodium chloride. A leached water-insoluble residue is obtained which is treated, preferably with the application of heat, with a mineral acid such as HCl which forms a water-soluble aluminum salt. The salt is then diluted with water to produce an aqueous solution of the aluminum salt and an insoluble residue comprising hydrated silica. The aluminum is then recovered from the salt solution.
None of the above processes relates to the simultaneous extraction of silica from coal ash.
U.S. Pat. No. 1,868,499 to Guertler describes a process for the recovery of alumina from silicious materials such as clay, leucite and silicious bauxite. The process comprises the steps of heating the silicious material with CaCl.sub.2 at 650-900.degree. C., treating the heated mixture with HCl to dissolve and separate CaCl.sub.2 for reuse and conversion of the aluminum to AlCl.sub.3, separating the non-gelatinous silicic acid precipitate, and purifying the AlCl.sub.3 solution and decomposing to form alumina. There is no indication of the purity of the extracted metal oxides, nor is there any indication that the recycled CaCl.sub.2 undergoes treatment.